Double-flush riveting is known in the art, the term double-flush riveting referring to a manual or machine accomplished manufacturing process where workpieces having opposed non-parallel surfaces are riveted to each other or to a frame member resulting in a smooth-skinned surface, both ends of the rivet being flush to the associated surface. In one prior art process, this is achieved by clamping the workpieces together between an upper clamp pad and a lower clamp pad. The upper clamp pad has a lower horizontal rigid surface which engages the upper or drill surface of the assembled workpieces. The lower clamp pad is a urethane member which can conform to the lower or shave surface of the workpieces even though the shave surface is at an angle to the upper or drill surface. An aperture is then drilled through the workpieces, the drill and aperture establishing a vertical workline. At the same time the upper or drill surface is countersunk to receive the head of a shear or flush head rivet. After the drilling operation is completed, the lower or shave surface of the workpieces is countersunk, the countersinking tool rotating about the vertical workline to establish a lower countersunk surface which is concentric with the centerline of the aperture. A shear head or flush head rivet is then inserted into the aperture and upset. The bucktail portion of the rivet which extends beyond the plane of the shave surface of the workpieces is then manually ground so that an aerodynamic surface is achieved.
A somewhat automated process has been developed in the past for accomplishing the above. Thus, the workpieces are clamped together, are drilled, and then the surfaces are countersunk with the centerline of the countersunk portions being co-extensive with the centerline of the aperture. After a rivet is inserted and upset, a milling tool or shaver is brought into engagement with the rivet bucktail to shave the same. To this end, the milling tool or shaver is mounted in a gimble-type clamp pad. In actual practice, it has been found while generally satisfactory results may be achieved when there is only a slight difference between the planes of the two surfaces, for example 2 to 3 degrees, that when the angle of the surfaces is greater it has not been practical to mill close to the shave surface of the workpiece without engaging the workpiece. In addition, it has been found through testing during the development of this invention that when the countersunk aperture is formed on the lower or shave side of the workpieces in the manner set fourth above that a good fill of the countersink cannot be achieved consistently when the shave surface is at an angle of more than 3 degrees with respect to the drill surface of the workpiece.